A packet switched streaming service PSS is currently being standardized for mobile environment by 3GPP (3rd Generation Partnership Project). An example of a communications system capable of streaming media (streaming video and/or audio) is shown in FIG. 1. The system comprises a streaming server 111 which is coupled to an IP-network (Internet Protocol) 104. The IP-network 104 may be, for example, the Internet or a service provider operator's intranet (an intranet network belonging to the operator's domain). The IP-network 104 is coupled to a core network 103 of a mobile communications network via a Gi interface. The mobile communications network also has a radio access network (RAN) 102 coupled to the core network 103. The radio access network 102 provides mobile communications devices 101 with access to the mobile communications network over an air-interface. Said access may be provided either by circuit switched means (circuit switched voice or data call) or packet switched means or both. In the following, GPRS (General Packet Radio Service) is used as an example of a packet switched means to communicate over the air-interface. Concerning GPRS, the generic term radio access network (RAN) is considered to comprise base (transceiver) stations (BS) and base station controllers (BSC).
In streaming media a sequence of ‘moving images’ (i.e. video) or sound (i.e. audio) or a sequence of ‘moving images’ together with sound (i.e. multimedia) is sent in a compressed form from the streaming server 111 to a mobile communications device 101 (hereinafter referred to as a client device 101). In contrast to the technique in which an entire media file has to be arrived at the client before it can be played, the streaming technique enables sending of media (video and/or audio) from the streaming server 111 to the client device 101 in a continuous manner and playing of the media as it arrives at the client.
When media is streamed in cellular mobile communications systems such as the system presented in FIG. 1, new problems specific to the mobile environment arise. These problems are mostly due to the different restrictions of mobile systems. One such problem arises in a cell reselection (i.e. handover) situation presented in FIG. 2.
In FIG. 2, a first base station BS1 of the radio access network 102 serves the area of a first cell 201 whereas a second base station BS2 of the radio access network 102 serves the area of a second cell 202. Before cell reselection, the client device 101 is served by the first base station BS1, i.e. the client device 101 has an active radio link with the first base station BS1. This situation is shown on the left side of the FIG. 2. After the cell reselection, the client device 101 is served by the second base station BS2. The radio link with the first base station BS 1, shown by a dotted line, is dropped. This situation is shown on the right side of the FIG. 2.
The cell reselection (CR) can be divided in three periods in time:    a) Pre-CR period    b) CR period    c) Post-CR period.
During the pre-CR period, received signal quality in the first cell weakens and the client device 101 starts cell reselection signaling. The client device 101 is able to receive streaming media via the first base station BS1 during that period. During the CR period, the actual cell reselection is performed. The client device 101 can not receive streaming media during that period. During the post-CR period, the client device 101 is able to receive streaming media via the second base station BS2.
The cell reselection may cause a long break in service. For example, a cell reselection (CR period) in a case where GPRS is used as radio bearer may take up to 30-40 seconds. This will affect an ongoing streaming session. For example, part of the packets carrying streaming media may be lost and/or freezing of the played media may occur at the client device 101. If the streaming media comprises a video stream, the freezing means that a still image will appear and stay for a period of time on a client device display before the streaming media is again received and can be played again. If the streaming media comprises an audio stream, the freezing means that no sound is played back (i.e. silence) until the CR period is over.
Accordingly, there is a need to find appropriate means for reducing the effects of cell reselection on an ongoing streaming session.